Solid polymer fuel cells (hereinafter sometimes referred to as “fuel cells”) are electric power generation devices that generate electrical power via a chemical reaction between hydrogen and oxygen, and hold great promise as a possible next generation energy source in fields such as the electrical equipment industry and the automotive industry. With respect to the polymer electrolyte membrane within these fuel cells, there has recently been attention to hydrocarbon-based polymer electrolytes, which are inexpensive and exhibit excellent heat resistance, in place of conventional fluorine-based polymer electrolytes.
As the hydrocarbon-based polymer electrolyte, if an ion-conductive polymer is used that is capable of forming a polymer electrolyte membrane in which a polymer segment having an ion-conductive component and a polymer segment having no ion-conductive component have exhibited microphase separation, then in the polymer electrolyte membrane, the polymer segment having an ion-conductive component has preferable properties, including forming a favorable ion conduction path to exhibit excellent ion conductivity, accordingly, investigations have focused mainly on the development of block copolymers having these two types of segments (for example, see JP-2003-31232-A, JP-2007-177197-A, and JP-2003-113136-A).
Examples of known methods of producing these types of ion-conductive polymers include a method in which a block copolymer composed of a segment having a site at which an ion-exchange group can be introduced and a segment having no such sites is first produced, and an ion-exchange group is then introduced into the block copolymer at the site at which an ion-exchange group can be introduced, a method in which a segment precursor having an ion-exchange group and a segment precursor having substantially no ion-exchange groups are both prepared, and the two segment precursors are then linked together to produce a block copolymer, and a method in which a monomer having an ion-exchange group is subjected to sequential polymerization with a segment precursor having substantially no ion-exchange groups to produce a block copolymer.
However, for an industrially produced block copolymer, producing a copolymer having substantially uniform properties in a stable manner is extremely difficult, which may act as obstacles from a quality control perspective. Accordingly, a series of cumbersome operations has conventionally been required in which, for example, fuel cell polymer electrolyte membranes are formed one by one from the ion-conductive polymers from various different production lots, the properties of the produced membranes are evaluated, and those ion-conductive polymers that would yield the required properties are then selected.
Further, in a different aspect, JP-2005-220193-A discloses an attempt to improve the properties of a polymer electrolyte membrane for a fuel cell (a proton conduction membrane) by mixing block copolymers that contain same structural units but have largely different ion-exchange capacities. However, there have been no previous reports of producing a fuel cell polymer electrolyte of stable quality by mixing a plurality of varieties of block copolymer.